(1) Field of the Invention
This invention relates to a method and a system for correcting errors of measurement which occur in tire uniformity inspection due to variations in the internal air pressure of a tire under inspection, by detecting a deviation of actual internal air pressure of a tire from a preset measuring pressure level to determine a correction coefficient from the value of deviation and multiplying an actually measured value of uniformity by the correction coefficient to wipe out the error of uniformity measurement due to a variation of the internal air pressure of the tire.
(2) Description of the Prior Art
According to the experiences and experiments in the past, a deviation .DELTA.P kgf/cm.sup.2 of a tire pressure from a predetermined measuring pressure level can have a large influence on the components of tire uniformity (mainly on RFV). For example, it is known that a deviation of 0.1 kgf/cm.sup.2 from a predetermined tire pressure causes a variation of 0.5 kgf in the measurement of a standard passenger car tire with a uniformity component at the level of 10 kgf. The variation occurs in different degrees depending upon the level of the uniformity component as shown in FIG. 3 which exemplifies tires of certain sizes.
The conventional tire uniformity inspecting machines usually have a precision reducing valve 1 provided in a main pneumatic circuit 2 as shown in FIG. 1 for maintaining a tire pressure with an extremely high stability. The pressure reducing valve 1 is capable of maintaining a given tire pressure with a stability on the order of 1/1000 kgf/cm.sup.2 as long as feed air is clean and it receives good maintenance and service. However, depending upon the degree of contamination or the moisture content of the feed air and the condition of maintenance, it easily loses stability, consequently giving rise to errors in the measured values of uniformity which are attributable to variations in the tire pressure.
In a tire uniformity inspecting machine, a tire 5 to be inspected is securely gripped between and rotated by a pair of upper and lower rims 3 and 4, while a load wheel 7 which is supported between a pair of load cells 6 is pressed against the rotating tire to measure variations in the radial force, namely, the uniformity of the tire. In such a uniformity measuring operation, it has been the conventional practice to control (or adjust) the tire pressure by the reducing valve 1, reading the pressure level on a pressure gauge 8 and supplying or discharging air by means of an electromagnetic change-over valve 9. The plant air (normally having a pressure level of 5-7 kg/cm.sup.2) which serves as an air source is reduced to a predetermined uniformity measuring pressure level (normally to 2 kgf/cm.sup.2) by the reducing valve 1. However, if moisture or dust deposits on the nozzle portion or valve seat of the pressure adjusting mechanism of the reducing valve 1, the measuring air pressure becomes unstable, as a result causing errors to the measured value of uniformity of the tire 7. According to the conventional procedure, a deviation of the uniformity measuring pressure from a predetermined pressure level due to malfunctioning of the reducing valve can be corrected only when the pressure level is constantly watched by an operator. However, in actual operations, the uniformity inspecting machine is used on a production line without constant watching of the tire pressure, accepting erroneous values of measurement in the absence of suitable correcting means.